I held off installing Office 2008/Mac till my new MacBook Pro arrived, because I was afraid I'd bog down my old iMac with the installation. Well, I installed Office 2008 on my new Mac today and so far everything has gone smoothly.
Installation was quick and easy, though I think after installation the software required about 4 major updates which made it all take a while longer.
I'm using Office 2007 on Windows XP, as well, and I've also used it on a Vista machine.
I like the way the new Mac version has kept nice, compact default views on all the major app pages. You can bring up all the nice tools and gadgets quickly and easily by just clicking on buttons for Gallery, Toolbox, Navigation, and so on. Those various toolbars fade in very quickly up top, giving you very easy access to templates and such without stealing too much additional screen real estate.
A lot of people gripe about Windows functionality. I don't really need high-powered usage on any of the apps. I'm setting up Word documents without too much formatting, PowerPoint decks using only minor tweaks of existing templates, and, well, come to think of it, I really don't ever deal with any huge spreadsheets in Excel. For average, everyday use, I think it's pretty adequate.
Also, I never loved Apple Mail. I just don't think it has enough time-management features. I'm glad to have more ability to set up appointments, link to actions and contacts, and track to-do's through Entourage.
Nevertheless, i also recently downloaded Open Office suite and am going to give that a thorough run-through in the next week or two. I'm more convinced than ever that open source is the way to go.